Early Cretaceous



dinosaur: the whole animal was just 39 centimetres long, and 24 centimetres of that was tail. Xu and Zhou classed it as the most primitive known dromaeosaur, which made it hugely relevant to the bird evolution debate.

The first microraptor fossils didn't have feathers, but two years ago Xu and Zhou stunned palaeontologists with six specimens of a closely related species, Microraptor gui, which had long feathers on its arms, hind legs and tail. The little dinosaur's arms were clearly too weak and short for powered flight, and so Xu suggested it was a glider that coasted down from the trees by spreading its forewings while holding its hind legs and tail together to form an aerofoil. In other words, the orthodoxy was only half right. Birds were indeed descended from dinosaurs, but flight evolved from the trees down.

Of course, microraptor itself couldn't be the ancestor of birds- the Yixian formation is younger than archaeopteryx. But perhaps the two shared a common ancestor. The discovery of microraptor's leg feathers prompted palaeontologists to re-examine other fossils. Amazingly, archaeopteryx and some other early birds turned out to have leg feathers that palaeontologists hadn't noticed before. So did pedopenna, an incomplete bird-like theropod from the Daohugou fossil beds of the Inner Mongolia region of China. Those beds are not well dated, but are somewhat older than the Yixian formation. BothXu and Zhou now argue that there was a glider stage on the road to powered flight.

Many other palaeontologists are sceptical or reserve judgement. "How you interpret it is open for discussion," says Currie. Tom Holtz of the University of Maryland is also cautious, as is Kevin Padian of the University of California, Berkeley. Martin, meanwhile, believes that microraptor is part of the story of flight, although he considers it and other dromaeosaurs to have been flightless birds rather than dinosaurs. ►

Yixian has already yielded a treasure trove of material. Some think Daohugou will prove just as fru itful

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